1. Field of the Invention
The present invention relates to an improved method of powder lining. More particularly, it relates to an improved method of powder lining where in a single step, or a multiplicity of steps, a uniform mixture of a synthetic resin powder and a specific carbon fiber, can be adhered onto a surface of a substrate, so as to attain a thick lining layer characterized by high mechanical strength.
2. Description of the Prior Art
Various lining methods, such as a sheet lining, dispersion lining, powder lining or the like, have been employed in the prior art. Preparation of a lining starting with a powder, however, has the advantages that no organic solvent is used and the adhesion of the lining layer to the substrate is excellent. Prior art powder lining methods include powder spraying, wherein a resin powder is sprayed onto a substrate, heated to a temperature higher than the melting point of the synthetic resin followed by subsequent baking of the coated layer; or electrostatic powder lining, wherein a resin powder is electrically charged and adhered onto a substrate by electrostatic attraction and then baked. In these conventional powder lining methods only thin layers can be formed in that if a thick lining is required, it is necessary to repeat the coating and baking steps several times. When the coating steps were repeated cracks were found to be easily formed in the lining layer due to stresses caused by the difference in thermal expansion between the synthetic resin layer and the substrate. Moreover, even though it is theoretically possible to obtain thick layers by the multiple application of the prior art techniques, in actuality in many of these techniques multiple applications was not feasible. For example, an ethylene-tetrafluoroethylene type copolymer is useful as an anticorrosive lining material, because of its high corrosion resistance and heat resistance. The thickness of the lining layer formed by the electrostatic powder lining of a ethylene-tetrafluoroethylene copolymer is about 100 - 150 microns. It is relatively difficult to repeat the coating to obtain a thicker lining in the conventional electrostatic powder lining, because of the electrical insulating characteristics of the first formed ethylene-tetrofluoroethylene copolymer layer. However the thickness of the layer formed by a single powder spraying step is about 100 - 300 microns. Despite the difficulty of multiple application, it is necessary to repeat the several coating-baking steps to attain a lining having a thickness of more than 500 microns, which is required for the corrosion resistance of lining layer to be satisfactory. In powder spraying the spray is conducted at a high temperature, because of this it is not desirable to repeat spraying operation.
There exists a need for a powder lining method which prepares relatively thick linings, having excellent mechanical properties in a single step which may, if desired, be repeated to prepare still thicker linings if desired.